The field of this invention is two stroke engines and, more particularly, relates to an air intake porting configuration that allows an increased cylinder bore and facilitates a corresponding power increase for a given exterior cylinder dimension.
In an effort to get more power out of a given frame size for a two stroke engine, one readily apparent way is to simply increase the bore of the cylinders. As a result, the power output increases by the square of the ratio of the new bore divided by the previous bore. The problem with doing this is that the throws on the crankshaft have given spacing, and the enlargement of the bore forces an increase in external dimensions of the cylinder. The existing block may also define limits to any desired increase of the bore, depending on the available spacing between the existing bores, for instance. The problem with expanding the bore size of two stroke engines is that air intake passages to the cylinder require a fair amount of space, because of their location. In the past, air was introduced through passages extending from the crank end of the power cylinder to the intersection of the intake ports with the main bore of the cylinder. Another way was to build an air chest into the engine block around the intake ports for the cylinder. However this method would substantially increase the size of the engine block, which increases the weight of the engine and may not be compatible with the given engine bay, for instance.
While a wholly new engine could be designed, such a process can be expensive and time consuming. It is clearly desirable if the bore size can be increased without major changes to the basic engine structure. In accordance with certain embodiments, the present invention provides methods and apparatus to increase the bore sizes of a given engine design without significant changes to the frame or crankshaft. The invention is put into perspective by a quick review of two stroke engine basics, shown in FIG. 1, and a comparison of the intake porting of a known design with that of the present invention shown in a comparison of FIGS. 2 and 3.
Referring to FIG. 1, a piston 10 having a rod 12 is disposed in a cylinder housing 14. The piston 10 also has a skirt 16 that defines a volume 18 around the rod 12. An inlet valve housing 20 includes a reed valve 22 that operates like a check valve. In the view of FIG. 1, the piston 10 is descending after a power stroke. Air that previously was drawn into housing 20 and past reed valve 22 is forced out of volume 18 as shown by arrows 24. That air that had been compressed under the skirt during the decision from the power stroke can, after the piston descends enough to expose the inlet ports 26, exit from under the skirt 16 to a passage 28 in the cylinder housing 14. The release of the pressurized air through passage 28 and through ports 26 scavenges out the remaining exhaust gasses in the cylinder 30 to exit through the exposed exhaust ports 32. After this happens, the piston 10 rises to close off intake ports 26 and exhaust ports 32. At that point, gas is injected through the gas injection valve 34, and the spark plug 36 ignites the mixture when the piston has nearly reached top dead center. Again the upward movement of the piston while the ports 26 and 32 are closed by the piston opens the reed valve 22 to allow more air to get sucked in. The cycles just described simply repeat as the engine operates.
FIG. 2 is similar to FIG. 1 and is placed on the same sheet as FIG. 3 to allow for an easy comparison of the differences therebetween. Referring first to FIG. 2, it can be seen that the presence of passage 28 leading to ports 26 along the outside of skirt 16 directly defines the size of the surrounding cylinder housing 14. In a given engine, any increase in the bore size B1 necessarily increases the size of the cylinder housing 14 and necessitates a redesign of the crank and engine frame, for example. These and other aspects of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings and the claims which define the full scope of the invention.